Human IL-21 Antibody - Avizakimab Biosimilar

Avizakimab background

Avizakimab (aka BOS-161721) is a therapeutic, humanized monoclonal antibody (mAb) that targets the human IL-21 (interleukin 21) [1]. This antibody was engineered to feature the M252Y/S254T/T256E triple mutation to enhance its bindind the the neonatal fragment crystallizable receptor (FcRn) and increase its half-life in plasma [1]. By binding to IL-21, Avizakimab prevents it from interacting with its receptor on the surface of immune cells, thus inhibiting its downstream functions, notably T cell and B cell proliferation and differentiation. Avizakimab is expected to induce T-cell exhaustion and tolerance in vivo [1].
IL-21 is a member of the IL-2/γc cytokine superfamily. It has pleiotropic actions on a wide range of immune (i.e. lymphoid and myeloid populations) and non-immune cell types (i.e. epithelial cells). IL-21 is referred to as a double-edged sword cytokine, playing key roles in both anti-tumor and anti-viral responses, as well as in promoting the development of autoimmune diseases (e.g., systemic lupus erythematosus and rheumatoid arthritis) [2].
Avizakimab has been clinically evaluated as an immunosuppressive agent for the treatment of systemic lupus erythematosus (SLE) [3].

References:

1. Hussaini A., et al., 2019. A Double-Blind, Phase I, Single Ascending Dose Study to Assess the Safety, Pharmacokinetics, and Pharmacodynamics of BOS161721 in Healthy Subjects. Clinical and Translational Science, 13(2):337.
2. Long D. et al., 2019. Clinical significance and immunobiology of IL-21 in autoimmunity. J Autoimmunity. 99:1.
3. NCT03371251 https://clinicaltrials.gov (accessed October 2025).